1. Field of the Invention
The present invention relates to a nanocomposite containing functional graphene and a polymer, the method of making and use of the nanocomposite, in particular for barrier membranes such as gas diffusion membranes.
2. Description of the Related Art
Over the past five decades, industrial scale ‘composite materials’ have been produced by adding numerous minerals and metals to thermosetting, thermoplastic and elastomeric polymers. Donnet, J. B., Nano and microcomposites of polymers elastomers and their reinforcement. Composites Science and Technology 2003, 63, (8), 1085-1088. As compared to the bulk polymers, these composites have shown moderate mechanical performance improvements in properties such as Young's modulus, tensile strength, abrasion resistance and storage modulus. However, recent advances in nanoscale particle synthesis have dramatically accelerated the growth of the composite industry. The capacity to synthesize and characterize atomic-level particles has produced a new generation of high-performance fillers. The incorporation of these sub-micron fillers in polymers, known as ‘nanocomposites’, has produced unparalleled performance improvements, easily outpacing earlier attempts. Commercial demand for nanocomposite materials has exploded. The possible applications for such materials cover a wide range of industries including food packaging, gasketing, automotive applications, portable electronic devices, etc.
Although this growth has been dominated by thermoplastic research, recently there has been an increased interest in elastomeric nanocomposites. Targeting industrial applications such as tire inner tubes, air springs and cure bladders, efforts have been made to improve the tensile and barrier properties of many commonly used rubbers. As such, work has begun to find cheaper alternatives to common specialty rubbers such as halogenated butyl rubber. In doing so, a number of filler materials have been investigated. Traditional choices include various minerals, carbon black and silicates. However, with advancing synthetic technologies, a number of promising new materials have emerged.
Particular interest is in the area of nanoparticle-filled polymer composites (NCs) in which the nanoparticle has dimensions comparable to those of the polymer chains, has a high aspect ratio of more than 100 and is uniformly dispersed in the polymer matrix. There are several filler materials that have been extensively studied for improvement of mechanical properties, electrical and thermal conductivity of polymer composites, for example, fractal agglomerated nanoparticles (silica and carbon black), carbon nanotubes (CNTs), inorganic clays and alumina silicate nanoplates.
Initial attempts at producing nanoparticle-filled polymer composites often resulted in materials with inadequate nanoparticle dispersion and degraded mechanical properties. Although often impractical for industrial applications, small-scale dispersion methods involving solvent- or monomer based processing have occasionally yielded NCs with multifunctional capabilities and improved mechanical properties. Several problems remain that affect the development of NCs with consistent properties that are viable for use in real world applications: (1) many of the nanoparticles used are expensive (e.g., CNTs); (2) often chemical or mechanical manipulations must be performed to achieve good dispersion that are impractical for large-scale commercial production; and (3) problems of the interfacial energy mismatch of inorganic nanofillers with hydrocarbon polymer matrix phases result in processing and mechanical property difficulties.
A significant amount of work has been done with nanoclays. Nanoclay-reinforced composites have shown enhancements in stiffness, glass transition temperature, barrier resistance, and resistance to flammability in a variety of polymer systems. Nanoclays are also high aspect ratio nanoplates that are, like graphene, derived from inexpensive commodity materials (clays) and thus appropriate for comparison with the projected graphene polymer composites of the present invention. The in-plane modulus of clays should be similar to that of mica, which is ˜178 GPa, significantly lower than the 1060 GPa value of graphene (value from graphite in-plane). Recent studies point out that the hydrophilicity of clays makes them incompatible with most polymers, which are hydrophobic. One approach is to render the clays organophilic through a variety of approaches (amino acids, organic ammonium salts, tetra organic phosphonium). Such clays are called “organoclays.” These materials have suffered from the cost of the added interfacial modifiers and the instability of these modifiers under processing conditions. In addition, it has been difficult to homogeneously disperse these organoclays in polymer matrices.
Carbon nanotubes (CNT) have also generated significant interest as nanofillers. They have good mechanical properties and large aspect ratios, and their surfaces should be more compatible with hydrocarbon polymers than clay-based nanofillers. As a nanofiller, CNTs have several limitations, one of which is their cost of production. Since they are made in a gas-phase process, the production costs are more expensive than solution-based processes operating at high density. The production of single wall carbon nanotubes (SWCNTs) requires the addition of metal catalysts that must be removed to produce pure SWCNT materials, or results in the presence of heavy metal contaminants in the final materials if not removed.
Graphite is a “semi-metal,” and recent efforts have demonstrated that extremely thin (few layers thick) nanoplates obtained from highly oriented pyrolytic graphite (HOPG) are stable, semimetallic, and have exceptional properties for metallic transistor applications.
Even though graphene sheets have the same sp2 carbon honey comb structure as carbon nanotubes (CNTs), until now, it has not been possible to effectively produce the highly dispersed, thin sheets needed to make graphene applications in polymer nanocomposites, in particular for gas barrier applications, possible.